Educational worksheet on chemical properties of matter, asking students to identify examples of chemical changes.

Problem Overview:

The task is to identify and color the phrases that illustrate examples of chemical properties having been changed, such that the substance cannot return to its original form. This involves recognizing chemical changes, which result in the formation of new substances with different properties.

Key Concepts:

1. Chemical Properties: These are properties that can only be observed by changing the chemical identity of a substance. Examples include the ability to corrode (rust), explode, burn, etc.
2. Chemical Changes: These involve the rearrangement of atoms to form new substances. The original substance cannot be recovered without further chemical reactions.
3. Physical Changes: These do not change the chemical identity of a substance. For example, dissolving salt in water or folding a piece of paper.

Solution:

We will analyze each phrase to determine whether it represents a chemical change or a physical change.

#### 1. Erosion
- Analysis: Erosion typically involves the physical breakdown of rocks and soil by natural forces like wind, water, or ice. No new substances are formed; it is a physical process.
- Conclusion: Physical change (not a chemical property change).

#### 2. Fireworks Explosion
- Analysis: Fireworks explosions involve chemical reactions where chemicals burn and release energy, light, and gases. New substances are formed, and the original chemicals cannot be recovered.
- Conclusion: Chemical change (illustrates a chemical property change).

#### 3. Bicycle Metal Rusting
- Analysis: Rusting is a chemical reaction where iron reacts with oxygen and water to form iron oxide (rust). The metal is permanently altered into a new substance.
- Conclusion: Chemical change (illustrates a chemical property change).

#### 4. Milk That Has Soured
- Analysis: Souring of milk occurs due to bacterial action, which breaks down lactose into lactic acid. This is a chemical reaction that produces new substances.
- Conclusion: Chemical change (illustrates a chemical property change).

#### 5. Baking a Cake
- Analysis: Baking a cake involves chemical reactions, such as the activation of baking powder or baking soda, which produce carbon dioxide gas and cause the cake to rise. The ingredients chemically combine to form a new substance (the baked cake).
- Conclusion: Chemical change (illustrates a chemical property change).

#### 6. Folding a Piece of Paper
- Analysis: Folding a piece of paper is a physical deformation; no new substances are formed. The paper can be unfolded to return to its original state.
- Conclusion: Physical change (not a chemical property change).

#### 7. Mold on Bread
- Analysis: Mold growth on bread is a biological process involving fungi that break down the bread's organic matter. This is a chemical reaction that produces new substances.
- Conclusion: Chemical change (illustrates a chemical property change).

#### 8. Frying an Egg
- Analysis: Frying an egg involves denaturation of proteins in the egg white and yolk, which is a chemical change. The egg cannot be returned to its original liquid state after frying.
- Conclusion: Chemical change (illustrates a chemical property change).

#### 9. Autumn Leaves
- Analysis: The change in color of autumn leaves is due to chemical processes within the leaves, such as the breakdown of chlorophyll and the production of other pigments. This is a chemical change.
- Conclusion: Chemical change (illustrates a chemical property change).

#### 10. Dissolve Salt in Water
- Analysis: Dissolving salt in water is a physical process where salt dissolves into ions in water. No new substances are formed, and the salt can be recovered by evaporation.
- Conclusion: Physical change (not a chemical property change).

#### 11. Sugar Turned to Caramel
- Analysis: Converting sugar to caramel involves heating sugar, which causes it to undergo a series of chemical reactions (caramelization). New substances are formed, and the original sugar cannot be recovered.
- Conclusion: Chemical change (illustrates a chemical property change).

Final Answer:

The phrases that illustrate examples of chemical properties having been changed so that they cannot be returned to their original form are:

- Fireworks explosion
- Bicycle metal rusting
- Milk that has soured
- Baking a cake
- Mold on bread
- Frying an egg
- Autumn leaves
- Sugar turned to caramel

These should be colored.

Final Answer:
\boxed{\text{fireworks explosion, bicycle metal rusting, milk that has soured, baking a cake, mold on bread, frying an egg, autumn leaves, sugar turned to caramel}}